Threaded plastic rod with mold burr for locking function



July 30, 1963 H. NAxMER 3,099,704

THREADED PLASTIC ROD WITH MOLD BURR FOR LOCKING FUNCTION Filed April 19, 1957 F192 Ia r-lIrF/g .f5 I 7 3K l l INVENTOHSI HUBERT NA! MER In the construction of electrical apparatus it is often necessary to force a plurality of structural elements together and to hold them together by tie rods. The laminated core of an electromagnetic device or a switch as is diagrammatically shown in FIGS. 1 and 2 of the drawing may be referred to as` examples. In that drawing FIG. l is a side View, partly in section, ot a switch composed of four identical units, and FIG. 2 is a top plan view of a switch unit. The problem with which the invention is concerned may be set forth iirst with reference to the latter iigure.

The switch apparent from FIGS. l and 2, which is shown in the left-hand part of FIG. l in a sectional view taken on line l--l of FIG. 2 and in the right-hand pant of FIG. l in a sectional View taken on line Ia-Ia of FIG. 2, whereas the latter is a sectional view taken on line II-II of FIG. l. The switch which is apparent trom said iigures has an operating shaft 1 provided with cams 2, which coact with rollers 3 acting by means of forks 4 on plungers `5, with which the jumper bridges 6 are connected, which can be lifted against the action of the closing springs 7 from the countercontacts t5 having terminals 9. The shaft 1 extends through the bottoms of casing parts 10, lila, ltlb, 10c and a cover 11 and may be rotatably supported in known manner. The shafts 12 of the rollers 3 extend through the prongs of the forks 4 and are guided in slots 13l formed in the casing part l-liic and in the cover 11. The casing parts 1ti-1ilc and the cover 11 are held together by four tie bolts 15. Such a switch construction is known. The number of casings 10 may vary within wide limits.

The invention is concerned more particularly with a special construction of tie rods of substantial length, illustrated by the bolts 15, which are found wherever several or many structural parts must lbe held together by tie rods, the length of which is a multiple of their diameter.

These tie rods consist in practice generally of metal and owing to their considerable length are not designed as headed screws but represent bolts having screw threads at both ends for threaded engagement with a nut with locking ring or with a double nut. S-uch a -bolt is apparent from FIG. 3.

The construction and arrangement of such metal t-ie rods in electrical devices must meet several requirements. In the case of a composite switch of the type shown in FIGS. 1 and 2 these requirements are as follows:

For cost reasons the tie rod consists in most cases of iron but in that case it must be given a surface protection, in most cases by Ielectro-cadniiumplating, electro-galvanizing, electro-nickelplating or the like. This is necessary because the switch (or another electrical apparatus) is often used in aggressive atmospheres or in moist rooms. The screw thread is either produced by machining or in a thread rolling machine. This requires in the iirst case tools subject to a particularly high wear and in the second case also an expensive machine. Similar requirements exist regarding the nuts, the screw thread of which must in any case be cuit. Thus the making of such bolts takes considerable time.

As to the arrangement of these bolts in the switch there is the requirement that the bolts must be sufficiently spaced from live parts, e.g. the terminals 9 or the jumper bridges dts z 6 in 'order rto ensure suthciently long electrical creep paths. Because the danger residing in the formation of creep paths can never be entirely precluded it is `often necessary to arrange the bolts in such a manner that they can be earthed. In many cases only the fultilling of these requirements necessitates the electrical apparatus to be made larger than would be required tor mechanical reasons or in View of the electrical power to be handled. In that case a certain alleviation can be provided by surrounding the bolts with lan insulating tube but that measure renders also the construction yof the device more complicated.

In order to avoid said disadvantages it is proposed according to the invention that the tension element consists exclusive-ly of -a thermoplastically deformable, electrically-insulating plastic, which is of high tensile strength and is moderately ilexible and tough `at normal temperatures, and is equal to or better than polyamide-base plastic in all said propenties. Superpolyamides have proved particularly satisfactory in that connection.

Superpolyamides have a melting point of about 200 C. and a strength which is suiicient for the present purposes in a temperature range of 20 to at least +l30 C. In that temperature range their tensile strength is 50G-600 lig/sq. cm., which is less than that of iron. This can easily be compensated by the fact that owing to the excellent insulating properties of said plastics no consideration has to be given Iany longer to creep paths. Thus the diameter of the plastic tie bolt can be increased and yet the bolt can be disposed more closely to the live parts. It must also be considered that tension members of iron are generally overdimensioned in a high degree.

Because such plastics are thermoplastic they can be made by injection moulding in multiple moulds, with head and screw thread, so that the separate forming of the screw thread is eliminated as well as any surface iinishing treatment. Owing to the elastic properties of such plastics, split locking rings and the like are eliminated and it is easily possible to connect the nut and bolt by local welding with the aid of a soldering iron or the like to permanently and perfectly secure them against undesired opening, particularly if the nut consists also of thermoplastic material. It is thus apparent that the Substitution of the usual tie rods of metal by tie rods of thermoplastically shaped suitable plastics ensures important advantages not only from manufacturing but also from the design aspect.

`Electrically non-conductive screws have already been proposed which consist of a metal core and a plastic covering which forms the screw thread and the screw head. In that known screws the metal core has a ribbed surface with which it is embedded in the plastic to ensure a torsion-resistant and tension-resistant connection therewith. That prior proposal relates to a countersunk screw rather than to a tie rod; whereas a countersunk screw cooperates with the nut thread, as a rule, over a length which is a multiple of the diameter of the pin, this is not the case with tie bolts acting in the manner of a screw or rivet. It was not recognized that tension-stressed structural elements of the present type can be made entirely of thermoplastics and the use of metal cores can be entirely eliminated and that it is also possible to provide tension elements which combine the advantages of perfect insulation with suiiicient strength and moderate plasticity, and which are formed with force-transmitting surfaces .which are subjected to shearing stress and are not larger or not substantially larger than in the case of a pure metal construction. -It is also not new to use rivets of thermoplastics; this applies also to electrical apparatus. This has always concerned only short rivet pins, the length of which was only a small multiple of their diameter so that a good heat soaking of the entire rivet shank and thus a good lilling of the rivet hole was achieved when the rivet pin was heated in order to form thel second rivet head. Highly stressed tie rods of large length (i.e., of a length which entirely precludes a continuous engagement of the rod periphery with the hole periphery and which is practically unlimited) and which consist of plastics are an entirely new structural element in electrical apparatus.

As is shown in FIG. 4 the present tie rod 20 may substantially take the form of a headed screw of metal. Contrary to metal tie rods the head 21 may now be provided because its formation in an injection mould involves no diculties and the mould may easily be adapted to the manufacture of ties of dilerent length by means of intermediate mould sections. The screw thread 22 is also produced in the mould and is used 'without any subsequent machining. The (exaggeratedly shown) burr 23, which is due to the mould is not removed, for reasons which Will be described hereinafter.

Another advantage of the plastic tie rods according to the invention consists in that when constructed as threaded bolts they can easily be made self-locking. 'lf the spontaneously formed burr 23, which is due to the split injection mould, is left on the bolt 20, as is apparent from FIG. 4, the burr will be deformed by the nut 24 when the latter is subsequently applied to the bolt. This has proved an excellent lock for the nut even if the same does not consist of plastics (the invention being not restricted to the use of plastic nuts). r'That burr need not be thicker than the one which is formed where good moulds are used. -It is obvious that such -a burr may also be provided in the nut itself.

I claim:

A tension-stressed tie rod interconnecting a plurality of aligned parts of an electrical apparatus, said rod having a length which is a multiple of its diameter and consisting of a thermoplastic resin at least equivalent to polyamide plastics in thermoplastic deformability, tensile strength, electrical insulating power, ilexibility and toughness at normal temperatures), said rod being injection molded in a mold having two mold parts with meeting faces and having at least one threaded end portion, and further comprising an axially extending ridge-like burr formed as an integral portion of said threaded end portion at the meeting faces of the mold and extending radially beyond the threaded crest.

References Cited in the le of this patent UNITED STATES PATENTS 1,062,000 Hahn May 20, 1913 1,640,433 Weldon Aug. 30, 1927 1,717,287 Warren et al lune 11, 1929 2,320,947 Martin et al lune 1, 1943 2,533,894 Podell Dec. 12, 1950 2,551,834 Ferguson May 8, 1951 2,568,640 Kindelberger Sept. 18, 1951 2,613,397 Borkland Oct. 14, 1952 2,624,916 Persak Tan. 13, 1953 2,664,023 Mugford Dec. 29, 1953 2,754,546 Mason et al. July 17, 1956 2,769,202 Thompson Nov. 6, 1956 2,890,845 Kiekhaefer lune 16, 1959 OTHER REFERENCES Product Engineering (published by the lPolychemicals Department, E. I. du Pont de Nemours and Co. Inc.) April 1954, pages 1, 2 and 3. Copy in Division 57. 

